1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery.
2. Description of the Prior Art
In recent years, shift of electronic devices into portable and cordless forms has been explosive, and accordingly, demands for compact, light-weighted, high energy density secondary batteries for use as a battery for driving such electronic devices have been increasing. Meanwhile, it has been proposed that a secondary battery is used not only for compact consumer devices but also for a power source for a device which need to have long-term durability, such as a power source for power storage, a power source for electric vehicles, etc. Broadening of the technical fields to which secondary batteries are applicable has been acceleratedly advancing.
Among various secondary batteries, nonaqueous electrolyte secondary batteries, especially, lithium ion secondary batteries, achieve higher voltages and higher energy densities. Therefore, the nonaqueous electrolyte secondary batteries have been expected for use as a power source for electronic devices, a power source for power storage, or a power source for electric vehicles.
The nonaqueous electrolyte secondary battery includes a positive electrode, a negative electrode, a separator, and nonaqueous electrolyte. The separator is interposed between the positive electrode and the negative electrode. The separator is mainly formed by a polyolefin microporous membrane. The nonaqueous electrolyte is contained at least in the separator. The nonaqueous electrolyte is liquid nonaqueous electrolyte prepared by dissolving lithium salt, such as LiBF4 or LiPF6, into an aprotic organic solvent. Nonaqueous electrolyte secondary batteries have been turned into practical uses, wherein the active material of the positive electrode is an active material which is high in potential relative to lithium and in which lithium ions are electrochemically intercalatable and deintercalatable (for example, LiCoO2, LiNiO2, LiMn2O4, LiFePO4) while the active material of the negative electrode is any of various carbon materials, such as graphite, or a metal oxide.
In such a nonaqueous electrolyte secondary battery, during charging operation, lithium ions from the active material of the positive electrode are intercalated between the crystal layers of the active material of the negative electrode, while during discharging operation, the lithium ions residing between the crystal layers of the active material of the negative electrode return to the active material of the positive electrode. Thus, charging and discharging of the nonaqueous electrolyte secondary battery cause the active material of the positive electrode and the active material of the negative electrode to expand or contract.
Specifically, the active material of the positive electrode releases lithium ions during charging operation and regains the lithium ions during discharging operation. Herein, the active material of the positive electrode exists in the form of a lithium oxide, a lithium phosphate, or a lithium sulfate. The crystal structure of the lithium oxide, the lithium phosphate, or the lithium sulfate has a robust framework, such as a layered rock-salt structure, a spinel structure, or an olivine structure. Therefore, expansion and contraction of the active material of the positive electrode due to intercalation and deintercalation of lithium are very small.
On the other hand, in the active material of the negative electrode, lithium ion is intercalated between the crystal layers of the active material of the negative electrode during charging operation so that the space between the crystal layers is expanded. Therefore, the charging operation causes the active material of the negative electrode to expand. In the case where an alloy is used as the active material of the negative electrode, the expansion of the active material of the negative electrode is very large.
When the active material of the negative electrode expands, there is a probability that the positive electrode and the separator are compressed. When the positive electrode and the separator are compressed, voids that exist in a mixture layer of the positive electrode and in the separator are collapsed. Since the voids contain the nonaqueous electrolyte, the collapse of the voids causes the nonaqueous electrolyte to be expelled out of the voids. As a result, as the cycles of charging/discharging are repeated over and over, the battery capacity can decrease (the cycle characteristics can deteriorate). Therefore, it is preferable to prevent expansion of the active material of the negative electrode during charging operation.
For the purpose of ameliorating the decrease in capacity which could occur along with repeated charging/discharging cycles, it has been proposed to mix Vapor Grown Carbon Fibers (VGCF) into the negative electrode (see Japanese Laid-Open Patent Publication No. 10-162811; hereinafter, this publication is referred to as “Document 1”). Document 1 describes that the vapor grown carbon fibers compensate for expansion and compression of the active material of the negative electrode to prevent deformation of the negative electrode. As a result, the decrease in capacity which could occur along with repeated charging/discharging cycles can be prevented.